Electrodeposition coating, or electrocoating, is widely used in the art for the application of polymer coatings to metal substrates. Electrodeposition baths usually comprise a principal film-forming resin, such as an acrylic or epoxy resin, with ionic groups that can be salted so that the resin can be dispersed or dissolved in an aqueous bath. Pigments (dispersed in resin pastes), dyes, flow control agents, and other additives are often included in the electrocoat bath.
For automotive or industrial applications where hard electrocoat films are desired, the bath also includes a blocked crosslinking agent that unblocks under appropriate conditions (e.g., with the application of heat) to react with functional groups on the principal resin and thus cure the coating.
One of the advantages of electrodeposition coating compositions and processes is that the coating composition can be applied to a variety of metallic substrates regardless of shape or configuration. This is especially advantageous when the coating is applied as an anticorrosive coating onto a substrate having a number of irregular surfaces, such as a motor vehicle body. In order to maximize an electrodeposition coating's anticorrosion effectiveness, it is important that the coating form a contiguous layer over all portions of the metallic substrate.
Two criteria for measuring the effectiveness of an electrodeposition coating for covering all portions of the substrate are throwpower and edge coverage. Throwpower measures the effectiveness of an electrodeposition coating at covering recessed or interior areas of a metal substrate. Edge coverage measures the effectiveness of an electrodeposition coating at covering the edges of a metallic substrate. Good throwpower and edge coverage are important in order to maximize an electrodeposition coating's anticorrosion effectiveness.
Electrodeposition coatings must often satisfy a number of other criteria as well. A high degree of smoothness is often desirable. For example, when the electrodeposition coating serves as a primer for a high-gloss topcoat, the primer layer must be very smooth in order for the topcoat to have a satisfactory appearance. It is also advantageous to exhibit stability over a range of pH.
It is therefore desirable to provide an electrodeposition coating composition that provides good throwpower and edge coverage, without compromising overall corrosion protection and smoothness.